Halogen bulb/LED cluster swap

Discussion in 'The Projects Forum' started by doug3460, Jan 26, 2009.

  1. doug3460

    Thread Starter Active Member

    Oct 19, 2008
    87
    0
    Hi all. Have been gone a bit but am back & still trying to blow myself up!:eek:

    Converting a 12VAC 10VA line to run LED cluster. Original had an M11 size 5ω halogen bulb. Should be no problem, right? 4 diodes & a capacitor (see attachments).

    Question: is there a good rule of thumb for determining an appropriate capacitor in simple circuits? For this one I chose a value based on research. I'd prefer to be able to compute a value without necessarily using tangent & cosine equations :rolleyes:.

    Suggestions?
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The capacitor will quickly charge to 16.8V.
    the value of the capacitor depends on how much ripple your circuit can tolerate.
     
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    If Vm is the peak voltage of the secondary and Vr is the ripple voltage then Vdc will equal:

    Vdc=Vm-Vr/2

    and

    Vr=Idc/(2*f*C) or Vr=Vdc/(2*f*C*R)

    Vr=peak-peak ripple voltage
    Idc=dc current the load draws
    Vdc=dc voltage across the load
    f=mains frequency
    C=filter capacitor value
    R=resistance of the load

    Note that the above equations give approximations of the voltages but close enough.
     
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